Along with an increase in the Internet traffic, a further large capacity of a main network is expected and a digital coherent receiver technology attracts attention as a technology for realizing large-capacity and long distance transmission.
The coherent reception relates to a receiver technology in which a received optical signal and local oscillation light of a receiver are mixed and electric field information (phase and intensity of light) of a reception optical signal is converted into an electrical signal and then it is demodulated. When the coherent reception is performed, noise tolerance is largely improved.
Further, in digital coherent reception, an electrical signal of extracted electric field information is quantized by an A/D converter to be converted into a digital signal, and it is demodulated by a digital signal process.
In general, in a waveform distortion such as wavelength dispersion and polarization mode dispersion generated in optical fiber communication, a distortion compensation is performed by using optical components such as a dispersion compensating module. As compared with the above, in the digital coherent reception, a wavelength distortion is compensated by using the digital signal process.
Accordingly, the digital coherent reception has a merit that when it is used, an optical component for a waveform distortion compensation is unnecessary, and therefore optical loss of the optical component is eliminated and cost is also reduced.
Further, the waveform distortion compensation by using the digital signal process is theoretically compensated in excess of critical use of a compensation capability in a normal optical component, and therefore waveform distortion tolerance is largely improved.
As a technology, a technology of suppressing a sampling phase shift by using an equalizer which compensates a waveform distortion is proposed. In addition, a technology of suppressing transmission degradation of an optical signal subjected to a band limitation on a transmission line is proposed.
Japanese Laid-open Patent Publication No. 07-235896
Japanese Laid-open Patent Publication No. 2010-147532
In an optical transmission system for developing a main network, wavelength division multiplexing (WDM) for multiplexing multiple different wavelengths and performing transmission is used.
An optical repeater installed on an optical fiber transmission line has not only an optical amplifier which performs optical-repeater amplification but also an optical filter. When a processing such as wavelength demultiplexing and wavelength multiplexing is performed by using the optical filter, an arbitrary wavelength channel according to communication requests is able to be inserted or branched in each optical repeater.
On the other hand, an optical signal filtered by the optical filter is subjected to narrowing of a signal spectrum. Therefore, in an optical network on which a number of optical repeaters are installed, since an optical signal transmitting through an optical fiber passes through optical filters over multiple stages, the narrowing of a signal spectrum appears prominently.
The above-described spectral narrowing brings about degradation in signal quality, and in addition, reduces a modulation frequency component of an optical signal. Therefore, there is reduced a sensitivity of a sampling clock shift detection circuit necessary for synchronizing a sampling clock frequency to an A/D converter with a modulation frequency of an optical signal. Further, reception quality is degraded due to reduction in jitter and wander tolerance of the optical signal.